Chronic nutrient additions can lead to drastic shifts in the plant community through time, both within tallgrass prairie in other grassland ecosystems worldwide. Nutrient addition experiments have answered many questions about patterns of diversity loss and community shifts; however, the level of nutrients which must be added to cause community shifts is unknown. To date, all nitrogen (N) addition experiments at Konza have added 10 g m-2 (e.g., NutNet Plots; Phosphorus (P) Plots; Belowground Plots), yet current rates of N deposition are one-tenth of that level. Even predicted rates of future N deposition in grasslands are not expected to exceed 5 g m-2 by the year 2050 and will likely be around 2 g m-2 for most of the US. This mismatch begs the question will 10 g/m2 affect grasslands the same way 2 or 5 g m-2 will? There are two main goals for this long-term experiment (1) to identify the nutrient threshold needed to drive plant community change with nutrient additions, and (2) to determine what factors underlie those threshold responses (build up of nutrients, mycorrhizal loss, invertebrate herbivory). Konza ChANGE is part of a multi-site experiment spanning grasslands on two different continents: North America – tallgrass prairie (KNZ) and shortgrass steppe (SGS), and China – three sites in Inner Mongolia. By including multiple grasslands, we expand our ability to make generalizations about how grasslands are affected by N additions, and whether thresholds, if they exist, vary with precipitation, natural nutrient availability, and species identity/composition. Research Questions: (1) Do ecosystems have N tolerance thresholds above which community composition will change, and does that differ between grassland types (i.e. mesic and xeric grasslands)? (2) Does adding a large amount of nutrients in one season result in an equivalent community change as adding a small amount over multiple years? (For example does 5 g m-2 for 6 years create the same community change as 30 g m-2 for 1 year or 15 g m-2 for 2 years?) (3) Will predicted levels of N deposition (2.5 g m-2 or 5 g m-2) elicit a community change? (4) Are there different thresholds for different plant functional types? For example, does a small amount of nutrients cause a decrease in N-fixing forb composition, while a larger amount of nutrients are necessary to reduce the abundance of the dominant C4 grasses? At what point does the community change state from a perennial grassland to the predicted annual forb community? Is it a gradual linear change or an abrupt transition? (5) Is decreased light availability due to increased ANPP the primary determinant leading to community shifts (like it is for species loss), or do other factors determine when nutrients cause a shift in community composition and structure such as interactions with invertebrate herbivory or loss of mycorrhizal symbionts.
To assess threshold responses of tallgrass prairie plant community composition and ecosystem function to a gradient of nitrogen addition.
Experimental Design: Forty-eight 5 x 5 m plots were established in June 2013 in R1B in 6 blocks, with 8 plots per block (Fig. 1). Each plot is divided into 4 2.5 x 2.5 m subplots. One subplot is used for core experiment measurements (species composition, ANPP, soil N availability, light availability, soil CO2 flux and other measurements). The remaining 3 subplots are set aside for future studies by the project PIs (e.g., insect herbivory monitoring and manipulations), as well as other Konza investigators. In 2013 pretreatment species composition and ANPP data were collected. In 2014 the nitrogen manipulations began. Each plot within a block receives a different nitrogen addition treatment: 0, 2.5, 5, 7.5, 10, 15, 20, or 30 g m-2 as slow time-release Urea. Plant community composition is measured in early and late growing season by estimating the aerial cover to nearest 1% in a permanent 1 x 1 m plot located within the core subplot of each 5 x 5 m plot. Productivity is measured by clipping all aboveground biomass within two 20 x 50 cm quadrats in the core subplot of each 5x5m plot at the end of each growing season.
Block A 39º 05’169” N 096º 33’309” W
Block B 39º 05’174” N 096º 33’292” W
Block C 39º 05’197” N 096º 33’267” W
Block D 39º 05’209” N 096º 33’253” W
Block E 39º 05’208” N 096º 33’270” W
Block F 39º 05’217” N 096º 33’258” W
Frequency of Sampling: Annually (ANPP), twice-annually (plant species composition), weekly to bi-weekly (soil CO2 flux, light measurements, soil nutrients and microbial communities)